“Conjugated” polymers are polymers containing a π-electron conjugated system along the main chain (or “backbone”), and have been known for some time to have utility as organic semiconducting materials. See, e.g., Organic Conductors, ch. 11, J. P. Farger, Ed. (New York, N.Y.: Marcel Dekker, 1994). Conjugated polymers include, for example, cis and trans polyacetylenes, polydiacetylenes, polyparaphenylenes, polypyrroles, polythiophenes, polybithiophenes, polyisothianaphthene, polyphenylenevinylenes, polythienylvinylenes, polyphenylenesulfide, and polyaniline. More recently, conjugated polymers have also been discovered to be useful as electroluminescent materials, i.e., as materials that emit light when excited by application of an electric current (Burroughs et al. (1990) Nature 347:539–541; May (1995) Phys. World 8(3):52–57). Accordingly, these polymers have been proposed for use in a variety of applications. For example, conjugated polymers may be used as the active material in semiconductor thin film devices such as light-emitting diodes (LEDs), transistors, photodetectors and solar cells. Conjugated polymers may also be used in electrochemical devices such as rechargeable batteries and light emitting electrochemical cells (both as thin films and in solution), as electrochemical sensors, and as electrical conductors (after being heavily doped).
The most promising application for conjugated polymers is in electroluminescent displays. Red, green, and blue emissions from conjugated polymers have all been demonstrated. Currently, red and green polymer LEDs are sufficiently bright, efficient, and stable for a number of display applications, whereas blue polymer LEDs lag in power efficiency and lifetime. Blue light emitting polymers are for the most part based on poly(paraphenylene) (PPP) and poly(fluorene), which is itself a type of poly(paraphenylene) except that every two neighboring phenyl rings are locked in one plane. Other blue light emitting polymers include alkoxy-substituted PPP, as disclosed by Yang et al. (1996), “Efficient Blue Polymer Light-Emitting Diodes from a Series of Soluble Poly(paraphenylene)s,” J. Appl. Phys. 79(2):934–939. Polyfluorenes with 9,9-dialkyl side groups are currently the best performing blue polymers, and are described, for example, in U.S. Pat. No. 5,900,327 to Pei et al. and U.S. Pat. No. 6,169,163 to Woo et al. Compared to red- and green-emitting polymers, however, blue-emitting polymers still lag in power efficiency (thus requiring a high operating voltage) and lifetime (i.e., the spectrum is not stable and tends to red-shift with time, heat, and operation). The problems stem from the dialkyl side groups, which reduce the polymers' glass transition temperature. The fluorene rings tend to aggregate, forming intraband defects that cause the color shift toward red.
There is accordingly a need in the art for blue-emitting polymers that have high blue luminescence, decreasing the required operating voltage for a blue polymer LED. Ideal polymers would also be conjugated, semiconductive polymers that are soluble in common organic solvents, particularly those used in semiconductor processing, but nevertheless retain semiconductivity, photoluminescence and electroluminescence efficiency, tensile strength, and thermal, chemical and photochemical stability. It would also be desirable to provide such polymers that may be easily modified so as to emit light in the red and green wavelength ranges.